Lightweight ceramic compositions which are strong and durable are in great demand for refractory structural and semi-structural applications. Lightweight ceramics are desirable because of their dimensional stability, insulating properties, non-combustibility and inertness to corrosive and abrasive environments.
Ceramics are made in lightweight form by a variety of processes which commonly comprise one of the following steps as the means of inducing porosity:
1. THE "BURNOUT" METHOD: THE INCORPORATION OF COMBUSTIBLE OR VOLATILIZABLE PARTICLES IN A MIXTURE OF CERAMIC RAW MATERIALS PRIOR TO FIRING;
2. "FIREBLOATING" A CERAMIC MATERIAL TO RELEASE A GAS SUCH AS STEAM OR AN OXIDE OF CARBON OR SULFUR;
3. BONDING LIGHTWEIGHT INORGANIC PARTICLES WITH A RELATIVELY DENSE CEMENTITIOUS MATRIX, SUCH AS CLAY- OR SILICA-BONDED BUBBLE ALUMINA; OR
4. DISPERSING OR DISSOLVING CERAMIC RAW MATERIALS IN A LIQUID, FOAMING THE LIQUID, AND THEN DRYING AND FIRING THE FOAM. In this method a preformed froth may be mixed with the dispersion or solution: a compressed or condensed gas may be incorporated into the dispersion or solution and allowed to expand; or finally a gas may be generated in the liquid medium by a chemical reaction.
An example of a burnout process is the manufacture of insulating firebrick from a mixture of clay, sawdust, and gypsum. Firebloating processes are used to manufacture lightweight clay masonry or insulation units, foamed glass, expanded perlite, and expanded shale aggregate.
Low density ceramics available commercially have disadvantages such as high cost, poor mechanical strength or difficulty in controlling density. Also, certain standard commercial processes for making lightweight ceramics particularly by the burnout or firebloating methods can discharge major amounts of carbonaceous or sulfur-containing air pollutants.